In some cases, a laser system operates near a self-damage threshold zone, resulting in an undesired outcome involving permanent irreversible damage to the laser system. The usual cause of laser damage is excessive optical power, resulting in optical electric field strengths that exceed the dielectric breakdown strengths of optics and surfaces and/or coatings employed in the optics, thus resulting in breakdown that amounts to permanent, irreversible damage. This excessive power, in turn, is typically caused by excess un-extracted gain, which not only leads to excessive optical power when extraction begins, but also can lead to self-pulsing that produces destructive output powers even with no input to the laser system present.
Lasers susceptible to optical damage may be operated in a continuous wave (CW) manner, in a Quasi-CW (QCW) manner, or pulsed, either repetitively or non-repetitively. Typically, risk for laser damage is greater in the pulsed systems because the intensities tend to be relatively higher, but the risk is not absent in CW lasers. Fiber lasers, for example, operate at relatively high intensities even in the CW case as a result of the small mode area corresponding to the active region.
In pulsed systems, the pulses are usually output on some sort of a schedule. However, circumstances can sometimes lead to a pulse not being produced at a scheduled time. A term of art for these is “missing pulses.” When a missing pulse occurs, the gain goes unextracted and continues to build as pumping continues. When the laser is operated near self-damage thresholds, even missing a single pulse could lead to damage. The pulse following the missing pulse would have been pumped for twice the interpulse period, rather than one times the period. This could lead to as much as twice the unextracted gain over the normal situation, requiring a safety margin of near 100% to avoid any damage. The safety margin would need to be even higher if more than one pulse in a row is missed.
Approaches to prevent damage caused from operating a laser system in the self-damage threshold zone have included detection of a missing-pulse(s) as well as administrative controls. However, these approaches can be limited in preventing damage in systems with agile waveforms.